A blast furnace is the most widely used equipment for producing pig iron, essentially composed of iron (92 to 95% by weight), carbon (3 to 5% by weight) and other elements in small amount, such as silicon, manganese, phosphorus, sulfur, etc.
This pig iron is then converted to steel in an oxygen converter, by injecting oxygen into the pig iron in the liquid state, in particular of oxidizing the carbon.
The steel obtained with then be refined and made to the desired grade (silicon steel, manganese steel, etc.) before being cast into ingots, slabs, blooms or billets.
A blast furnace is essentially fed with iron ore (in general 1.3 to 1.6 tonnes per tonne of pig iron produced) in the form of agglomerates or pellets, introduced via the top of the blast furnace, with coke (between 250 and 500 kg per tonne of pig iron), also introduced via the top, pulverized coal injected into the tuyeres, the injected amount possibly varying between 0 and 250 kg per tonne of pig iron, or with any other fuel, such as natural gas, fuel oil, coking gas, plastics, and with air, also called “wind”, with a flow rate that may vary from 800 to 1200 Sm3 per tonne of pig iron produced, the air being enriched with oxygen or not, this enrichment possibly varying from 0 to about 15% by volume, i.e. 0 to 150 Sm3 of oxygen per tonne of pig iron produced.
This blast furnace produces mainly pig iron, slag (200 to 400 kg per tonne of pig iron produced), which slag may then be utilized in various applications, and gases, containing in particular nitrogen (40 to 60% by volume), carbon monoxide CO (20 to 25% by volume), carbon dioxide CO2 (20 to 25% by volume) and hydrogen (1 to 7% by volume).
Various other elements with a content of less than 1% may also be produced.
The gas or gas mixture output by the blast furnace is generally recovered and used for its thermal value, either by direct exchange, in order to lower its temperature and increase that of the gas or fluid with which it is in heat exchange, or by combustion, for example CO with oxygen so as to produce additional heat.
The blast-furnace wind, whether enriched with oxygen or not, is injected at the base of the blast furnace via tuyeres that are distributed all around the circumference of the blast furnace.
This wind is injected under a pressure that may vary from 1 to 7×105 Pa so as to overcome the pressure drop in the blast furnace and the pressure at the top of the charge in the blast furnace.
The air flow rates required are very high, varying from 5000 Sm3/hour for very small blast furnaces (for example those seen at the present time particularly in China) up to 500 000 Sm3/hour for very large industrial blast furnaces.
To bring the ambient air to this pressure, very powerful air compressors or “blowers” are used, one (or more) blowers being dedicated to one blast furnace.
In a factory producing pig iron and having more than one blast furnace, it is general practice when having n blast furnaces to use at least n+1 blowers and sometimes n+2 blowers, so as to ensure continuous pig iron production when one of these blowers possibly breaks down (or has to be stopped for maintenance or any other reason).
Now, the redundant blowers (also called second blowers) which are redundant relative to the number of blast furnaces, are generally mounted alongside the other blowers in operation and are in a stand-by position, ready to be started so as to ensure continuity of pig iron production, even when an air pressure and/or flow rate on a blower at a predetermined value below which it is necessary to replace this blower with one of the stand-by blowers, is detected.
In general, to enrich the air wind with oxygen, one or more large-capacity oxygen production units, generally cryogenic air separation units producing oxygen of industrial purity, that is to say generally a purity greater than 80 vol %, preferably greater than 90 vol %, more preferably greater than 95 vol % and sometimes of purity greater than 99 vol %, are provided on the pig iron production site close to the blast furnaces or are connected thereto via lines.
The increase in oxygen requirement of a pig iron production site may arise either in the case of an increase in pig iron production in the existing blast furnaces, or by addition of one or more new blast furnaces on the site, or by increase in the specific oxygen consumption in each blast furnace, as a result, for example, of the addition of more fuel, such as coal, natural gas, fuel oil, coking gas, plastics, etc. (this addition generally takes place in the tuyeres). This increase may result from the use of oxygen for another technical objective, such as for example the enrichment of air dedicated for cowper preheating.
In this case, the increase in oxygen requirement may result in the construction of a new oxygen production unit, whether a cryogenic air separation unit or a unit producing oxygen by what are called VPSA processes.
When it is necessary to make such an investment in a new air gas separation unit, taking into account the high cost of such a unit, it may prove necessary or preferable to use components already existing on the site.
The method according to the invention involves this problem thus posed.